|Locale||Pennsylvania, Delaware, and Maryland|
|Number of tracks||1-2|
The Philadelphia Subdivision is a railroad line owned and operated by CSX Transportation in the U.S. states of Pennsylvania, Delaware, and Maryland. The line runs from Philadelphia, Pennsylvania southwest to Baltimore, Maryland along a former Baltimore and Ohio Railroad (B&O) line.
At its north end CP NICE, near the Nicetown section of Philadelphia, the Philadelphia Subdivision becomes the Trenton Subdivision; The south end of the Philadelphia Subdivision is near Bay View Yard, where the Baltimore Terminal Subdivision begins.
The line was built by the Baltimore and Philadelphia Railroad in Pennsylvania and as a branch of the B&O Railroad in Delaware and Maryland. The line began full operation in 1886. North of Philadelphia, the B&O used the lines of the Philadelphia and Reading Railway to reach the New York City area. Passenger train service on the Philadelphia Subdivision was led by the Royal Blue, its flagship train. The B&O ceased operation of passenger trains on the subdivision in 1958, and since then the line has been used only for freight trains.
In the 1970s and 80s the line passed through leases and mergers to CSX.
Communities such as Aberdeen, Maryland are making an effort to preserve the former Baltimore and Ohio railroad stations.